Process for improving the reactivity of zinc particles in producing sodium dithionite from zinc dithionite

ABSTRACT

The present invention relates to a process for preparing a zinc dithionite solution wherein the process comprises reacting an aqueous slurry of zinc particles with sulfur dioxide in presence of a promoter compound. The zinc particles can either be a commercial particulate zinc material or a particulate zinc material prepared by electrolyzing an aqueous alkaline suspension or slurry containing zinc based species. The addition of the promoter compound significantly increases the conversion of zinc to zinc dithionite and significantly reduces the cycle time of the reaction. The promoter compound is selected from the group consisting of sodium hydroxide and metal compounds. The promoter metal compounds are selected from the group consisting of an alkali metal hydroxide, zinc oxide, zinc sulfite, zinc bisulfite, zinc metabisulfite, an alkali metal zincate, an alkali metal sulfite, an alkali metal bisulfite, and mixtures thereof.

FIELD OF THE INVENTION

[0001] The present invention relates to a chemical process for preparing sodium dithionite. More particularly, the present invention relates to a process for making zinc dithionite as an intermediate step in the production of sodium dithionite.

BACKGROUND OF THE INVENTION

[0002] Dithionites can be manufactured by the reduction of sulfites, bisulfites, or sulfur dioxide with reductants, e.g., sodium or zinc. These manufacturing processes have been reviewed by A. Janson and coworkers in Ullmanns Encyklopadie Technischen Chemie, 3rd edition Volume 15, published by Urban & Schwartzenberg, of Munchen-Berlin, 1964, in pages 480-488. A typical older process involved the reduction of liquid sulfur dioxide with an aqueous slurry of particulate zinc material is disclosed in British Patent 1,145,824. These references are hereby incorporated by reference. In the chemical and metallurgical industries, particulate zinc material is used as a reducing agent, in the manufacture of hydrosulfite (dithionite) compounds for use in the textile and paper industries, and to enhance the physical properties of plastics and lubricants. Zinc dithionite is an intermediate in the manufacturing of sodium dithionite.

[0003] Zinc metal is generally produced by pyrometallurgical or electrochemical process. Particulate zinc material and powder are particulate forms of zinc.

[0004] The terms dust and powder have been used indiscriminately to designate particulate zinc materials. The term particulate zinc material designates material produced by condensation of zinc vapor, whereas zinc powder indicates the product obtained by atomizing molten zinc. Particulate zinc materials are manufactured in various size ranges and a typical commercial dust has an average particle diameter between 4 and 8 μm. Usually, dusts are screened to be essentially free of particles coarser than 75 μm (200 mesh). The particle size distribution for commercial zinc powders typically range from 44 to 841 μm. (325-20 mesh).

[0005] In the manufacture of dithionite from zinc, the zinc reactivity of a particulate zinc material is a critical parameter in determining the ultimate commercial conversion of the zinc material into the dithionite form. The zinc reactivity of particulate zinc material is largely a function of the origin of the zinc as well as the processing the zinc undergoes. Particulate zinc material manufacturers use different atomization processes such as air atomization, nitrogen atomization, etc., and various zinc ores may have different compositions. Thus, particulate zinc materials from different suppliers typically contain different amounts of metallic zinc, zinc oxide, and small concentrations of other metallic and nonmetallic impurities. For example, the concentration of zinc oxide found in zinc material from commercial suppliers typically is less than about 1 weight percent. Other metallic impurities which could include compounds of lead, iron, cadmium, indium, bismuth and copper are typically present in amounts less than about 0.1 weight percent. Furthermore, particle size and particle size distribution of particulate zinc materials from different suppliers may be different. Because of these differences, particulate zinc materials from different suppliers can show significantly different zinc reactivity with sulfur dioxide.

[0006] Efforts have been made to prepare zinc powder by electrolyzing a solution of zinc based compounds in strong alkaline solutions. I. Orszagh and V. Vass (Hungarian Journal of Industrial Chemistry, Vol. 13, pp. 287-299 (1985)) disclose an electrochemical method to regenerate zinc mud obtained during sodium dithionite production. The zinc mud obtained in sodium dithionite production, contains 50-60% water and 30-35% zinc, and is in the form of zinc hydroxide. In Orszagh et al., the zinc mud was dissolved in caustic, the resulting zinc solution is clarified and electrolyzed, and the recovered zinc powder is used to reduce sulfur dioxide. According to Orszag et al., the process requires a number of labor intensive operations such as lifting of the electrodes, removal of the zinc powder by sedimentation and filtration, drying, grinding and sizing, etc. These labor intensive steps combine to make the process of Orszagh et al. economically unattractive. For example, zinc particles so produced were washed with water (20 to 60 ml water per gram of the zinc), dried, ground, and sieved to provide zinc particles with the desired particle size distribution. Furthermore, the Orszagh et al. process requires a divided electrolytical cell, which is operating and capital cost intensive. Such electrolytic cells require high cell voltage at a low current density which contributes to high capital and operating expenses. In summary, this process is economically unattractive as an alternative to the currently used particulate zinc material based technology. Because of these disadvantages, there is a strong need for an improved cost effective process to recycle zinc oxide byproduct in the production of zinc dithionite.

[0007] U.S. patent application Ser. No. 09/776,518 (filed Feb. 2, 2001) discloses an electrochemical process for preparing zinc powder which involves: a) providing to an electrochemical cell a basic solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the basic solution prepared by dissolving the zinc oxide or the other zinc compound in an aqueous 2.5 to 10.0 M base solution; and b) passing current to the cell at a current density of about 10,000 to about 40,000 A/m² for a time period sufficient to electrochemically reduce the zinc oxide to zinc powder, wherein the electrochemical process has a current efficiency of at least 70% and is substantially free from electrode corrosion.

[0008] U.S. patent application Ser. No. 09/776,644 (filed Feb. 2, 2001) discloses a continuous electrochemical process for preparing zinc powder which involves: providing to an electrochemical cell a solution or suspension in an aqueous 1.25 Molar to 10.0 Molar base solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the solution or suspension containing at least 2 millimoles of solubilized zinc based species per 100 grams of electrolyte; and b) passing current to the cell at a current density of about 500 to 40,000 A/m², for a time period sufficient to electrochemically reduce the solubilized zinc based species to zinc powder, while continuously or intermittently adding a sufficient amount of the zinc oxide or the other zinc compound to the cell to maintain the concentration of the solubilized zinc based species at a level of at least 2 millimoles per 100 grams of electrolyte and continuously or intermittently removing at least a portion of the zinc powder formed; wherein the electrolyte includes the aqueous base solution and the zinc oxide or the other zinc compound.

[0009] U.S. patent application Ser. No. 09/965,157 (filed Sep. 27, 2001), provides a process for preparing a solution of zinc oxide in an aqueous base, said process comprising diluting a more concentrated solution of zinc oxide in aqueous sodium or potassium hydroxide to produce a resulting dilute solution of zinc oxide having a concentration of zinc oxide that is higher than that obtained by dissolving solid zinc oxide in aqueous sodium or potassium hydroxide, wherein the concentration of the aqueous sodium or potassium hydroxide used for dissolving the solid zinc oxide is substantially the same as the concentration of the aqueous sodium or potassium hydroxide in the resulting dilute solution of zinc oxide, and wherein the concentration of the aqueous sodium hydroxide in the resulting dilute solution ranges from 5 wt % NaOH to about 35 wt % NaOH, and the concentration of the aqueous potassium hydroxide in the resulting dilute solution ranges from 10 wt % KOH to about 55 wt % KOH. This process provides a way to prepare an alkaline solution containing a greater concentration of zinc based species and hence can be electrolyzed to prepare particulate zinc material.

[0010] U.S. patent application Ser. No. 10/015,185 (filed Dec. 7, 2001) provides a low corrosion electrochemical process for preparing zinc metal which comprises electrochemically reducing an aqueous basic solution or slurry of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, wherein the electrochemical process is carried out in an undivided electrochemical cell, and wherein air or nitrogen is bubbled in through the solution or slurry of zinc oxide or other zinc compound during the electrochemical process.

[0011] U.S. patent application Ser. No. 10/109,443 (filed Mar. 28, 2002) provides a process for preparing zinc dithionite comprising reacting zinc metal with sulfur dioxide, wherein the zinc metal used in the reaction is produced by electrochemical reduction in an undivided electrochemical cell of an aqueous alkaline slurry or solution of zinc oxide or any other zinc compound that reacts with aqueous base to form zinc oxide. Current efficiency for the formation of zinc dithionite can be obtained by multiplying current efficiency for the zinc formation with conversion efficiency of zinc to zinc dithionite. Current efficiency of zinc formation is quite high (such as>90%). If reactivity of the zinc formed is similar to the particulate zinc material currently used in manufacturing zinc dithionite, conversion efficiency of zinc to zinc dithionite is also quite high. This suggests that the current efficiency for the zinc dithionite formation should also be quite high. Current efficiencies for the zinc dithionite formation in an undivided cell varied from 73% to 85% depending on several operating variables including the nature of the electrodes, the current density, and the concentration of sodium hydroxide.

[0012] It is highly desirable to develop a process where the use of less reactive zinc particles will lead to a lower reaction time and/or a higher yield of zinc dithionite. Less reactive zinc particles can either be a commercial particulate zinc material or a particulate zinc material prepared by electrolyzing an aqueous alkaline solution containing zinc based species at magnesium, stainless steel, copper, etc. based cathodes. The present invention affords such a process.

SUMMARY OF THE INVENTION

[0013] According to the present invention, the zinc dithionite solution can be prepared from either zinc particles produced as a commercial particulate zinc material or a particulate zinc material prepared by electrolyzing an aqueous alkaline suspension or slurry containing zinc based species at conversions which are unexpectedly higher than earlier processes and at overall cycle times which are surprisingly shorter than previously known or possible before the present invention, significantly reducing the cost and the complexity of the overall process for the production of sodium dithionite. We have unexpectedly learned that the addition of a promoter compound which comprises a metal hydroxide, sulfite, bisulfite, metabisulfite—in particular, a promoter compound comprises at least one compound selected from the group consisting of an alkali metal hydroxide, zinc oxide, zinc hydroxide, alkali metal zincate, zinc sulfite, zinc bisulfite, zinc metabisulfite, alkali metal sulfite, alkali metal bisulfite and mixtures thereof.—to the reaction mixture before or after starting the addition of sulfur dioxide to the zinc dithionite reaction zone increases reactivity of the zinc particles. The term alkali metal includes sodium, potassium, lithium, and mixtures thereof. The preferred alkali metal is sodium. Because of the increased reactivity of zinc particles, sulfur dioxide can be added at a constant high rate for a longer period of time and results in a significantly increased yield of zinc dithionite, hence sodium dithionite, and improves the conversion of zinc particles.

[0014] In one embodiment, the present invention is a chemical process for preparing a zinc dithionite solution which comprises reacting an aqueous slurry of zinc metal with sulfur dioxide in presence of an effective amount of a promoter compound selected from the group consisting of sodium hydroxide, zinc oxide, zinc sulfite, zinc bisulfite, zinc metabisufite and mixtures thereof.

[0015] In another embodiment, the present invention is a process for preparing a sodium dithionite product solution comprising:

[0016] a) admixing a zinc oxide feed stream with a water stream comprising caustic to provide a zinc oxide slurry and passing the zinc oxide slurry to an undivided electrochemical reaction zone and therein electrolyzing the zinc oxide slurry to provide a zinc metal suspension;

[0017] b) recovering a zinc metal stream from the zinc metal suspension;

[0018] c) washing the zinc metal stream to provide an aqueous zinc metal suspension;

[0019] d) contacting the aqueous zinc metal suspension with a sulfur dioxide stream in the presence of an effective amount of a promoter compound in a zinc dithionite reaction zone to provide a zinc dithionite solution;

[0020] e) contacting the zinc dithionite solution with a caustic solution in a sodium dithionite reaction zone to provide a sodium dithionite effluent stream comprising sodium dithionite, water and a solid zinc oxide material;

[0021] f) separating the solid zinc oxide material from the sodium dithionite effluent to provide a recovered zinc oxide stream and the sodium dithionite product stream;

[0022] g) washing the recovered zinc oxide stream to provide a washed zinc oxide stream;

[0023] h) combining at least a portion of the washed zinc oxide stream with the zinc oxide feed stream prior to step(a), and optionally,

[0024] i) admixing a portion of the washed zinc oxide stream with the aqueous zinc metal suspension prior to step(d).

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a simplified flow diagram of the process of the present invention.

[0026]FIG. 2 is a simplified process flow diagram of the present invention illustrating a zinc metal recovery scheme.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Reactivity of the zinc particles is of major commercial importance because the reactivity determines the rate at which zinc particles will react with sulfur dioxide. When reacted with sulfur dioxide, less reactive zinc particles will lead to either an increased reaction time and/or a lower conversion and hence a lower yield of zinc dithionite. Increased zinc dithionite reaction time, leads to an increase in cycle time for producing zinc dithionite, and thereby results in significantly increased production costs. A lower conversion will lead to the production of zinc oxide containing a greater amount of metallic zinc particles. This is undesirable from byproduct quality as well as safety considerations if zinc oxide is not used as a raw material for preparing zinc particles. For example, if commercial zinc dust is used as a raw material and byproduct zinc oxide is sold for other uses, presence of metallic zinc particles is highly undesirable. This is especially true if concentration of metallic zinc particles is relatively high. In contrast, if particulate zinc material is prepared by electrolyzing a solution of zinc oxide in an aqueous alkali solution, presence of metallic zinc particles in zinc oxide byproduct is not a serious problem. In fact, presence of zinc particles in zinc oxide will lead to an apparent increase in the current efficiency for the zinc dithionite formation. The lower yield of zinc dithionite leads to a higher production cost and hence is highly undesirable. Thus, it is highly desirable to develop a process in which the reactivity of less reactive zinc particles is increased either to achieve a higher yield of zinc dithionite or a lower reaction time and thereby provide a lower cycle time for the zinc dithionite formation reaction.

[0028] Overall rate of reaction of zinc particles with sulfur dioxide depends on several factors such as:

[0029] 1. The reactivity of metallic zinc particles.

[0030] 2. The rate at which reaction mixture is stirred and mixed.

[0031] 3. Ratio of the amount of water to the amount of zinc particles used.

[0032] 4. The rate at which sulfur dioxide is added to the reaction mixture. In order to maximize the production rate, sulfur dioxide addition rate which is generally maintained at a constant rate is maximized. During the addition of sulfur dioxide, pH of the reaction mixture is maintained above 3.3 and temperature is maintained below 54° C. to minimize decomposition of the zinc dithionite solution. To increase conversion efficiency of zinc particles, sulfur dioxide addition rate is lowered several times in a series of constant rate increments or steps towards the end of the reaction to maintain control of the pH of the reaction mixture above about 3.3. This adjustment of the sulfur dioxide addition rate during the reaction makes the process more labor intensive and increases reaction time significantly. It is highly desirable to achieve a higher yield of zinc dithionite and a higher conversion of zinc particles when reactions are carried out with a constant high addition rate of sulfur dioxide. In order to maximize sulfur dioxide addition rate, reaction mixture is adequately stirred and mixed. Sensitivity of the reaction rate to the rate of stirring and mixing increases as the ratio of the amount of water to the amount of particulate zinc material used is decreased.

[0033] Zinc particles used in the present invention can either be a commercial particulate zinc material or can be prepared by electrolyzing a suspension or a solution of zinc oxide in an aqueous alkaline solution. Electrolysis can be carried out according to the processes described U.S. patent application Ser. No. 09/776,518 (filed Feb. 2, 2001), Ser. No. 09/776,644 (filed Feb. 2, 2001), Ser. No. 09/965,157 (filed Sep. 27, 2001), and Ser. No. 10/015,185 (filed Dec. 7, 2001), which are hereby incorporated by reference. According to the process of the present invention while converting zinc metal produced electrochemically, it was surprisingly discovered that adding to the electrolytic cell at least a portion of insolubles recovered from the zinc dithionite solution, which are believed to comprise zinc sulfide, resulted in a significant improvement in the current efficiency or yield of both the zinc dithionite and the sodium dithionite. As shown hereinbelow by way of example, the operation of the electrolytic cell without the addition of the insolubles resulted in a zinc dithionite yield of about 69 mol-%, and with the addition of the insolubles to the electrolytic cell, the yield of zinc dithionite rose to about 89 mol-%. When electrochemically produced zinc is the only source of zinc included in the zinc dithionite reaction mixture, the current efficiency is equal to the yield of zinc dithionite.

[0034] In processing commercial zinc dusts it was unexpectedly discovered that adding up to about 6 weight percent (weight percent of the amount of zinc particles used) of sodium hydroxide to the zinc dithionite reaction zone resulted in significantly increased yield and significantly reduced cycle time. As used herein the term cycle time refers to the time required to convert a batch of zinc dust into zinc dithionite by reaction with sulfur dioxide in the zinc dithionite reaction zone. A reduction in cycle time provides the benefit of increased capacity or improved conversion. It was unexpectedly discovered that the kinetics of the reaction was altered by the addition of sodium hydroxide. In prior operations of the zinc dithionite reaction zone, it had been required to lower the sulfur dioxide addition rates as the reaction approached the terminal pH to avoid loss in yield of zinc dithionite. The addition of sodium hydroxide permitted the reaction to approach completion at reduced cycle time, without decreasing the sulfur dioxide addition rate and without significantly decreasing the yield of sodium dithionite produced downstream in the sodium dithionite reaction. Furthermore, the quality of the zinc oxide byproduct produced in this manner was improved. Generally, one skilled in the art would expect that sodium dithionite is less stable than zinc dithionite and would expect that the addition of sodium hydroxide to the zinc dithionite reaction zone in the presence of sulfur dioxide would produce some sodium dithionite. The sodium dithionite would be expected to decompose early in the reaction to produce sodium thiosulfate. However, it was observed that the formation of sodium thiosulfate was significantly reduced compared to production of thiosulfate in conventional zinc dithionite reactions without the addition of sodium hydroxide.

[0035] The addition of a promoter compound such as alkili metal hydroxide, metal oxides (particularly, zinc oxide), sulfites, bisulfites and metabisulfites to the zinc dithionite reaction zone was found to significantly reduce the conventionally observed initial sharp drop in pH which occurred as sulfur dioxide was introduced to the zinc dithionite reaction zone. A reduction in pH during the zinc dithionite reaction leads to the decomposition of the desired species and results in the increased production of impurities such as sulfur and sulfides, etc. Thus, by the addition of a promoter to the zinc dithionite reaction zone, the rate of the reduction of the pH is reduced and results in the following benefits: the yield of zinc dithionite is increased, the cycle time is reduced, the amount of impurities is reduced, and the quality of the zinc oxide byproduct is improved. Finally, the production costs for producing zinc dithionite are reduced by eliminating the step-wise reduction in the rates of sulfur dioxide addition. This unexpected result also permitted the zinc dithionite reaction to be carried out in a series of partial conversion cycles, wherein the zinc dithionite reaction is partially converted, the reaction is terminated, and the zinc dithionite reaction zone effluent is filtered to separate a zinc dithionite solution, from unreacted zinc suspension and any insolubles. In a succeeding cycle, the unreacted zinc suspension and any insolubles are returned to the zinc dithionite reaction zone, and the reaction resumed.

DETAILED DESCRIPTION OF THE DRAWINGS

[0036] The process of the present invention is hereinafter described with reference to FIGS. 1 and 2 which illustrate various aspects of the process. It is to be understood that no limitation to the scope of the claims which follow is intended by the following description. Those skilled in the art will recognize that these process flow diagrams have been simplified by the elimination of many necessary pieces of process equipment including some heat exchangers, process control systems, pumps, filtration systems, etc. It may also be discerned that the process flow depicted in the figures may be modified in many aspects without departing from the basic overall concept of the invention.

[0037]FIG. 1 illustrates one embodiment of the present invention which comprises the conversion of commercial zinc metal dust to sodium dithionite employing the improved process for the intermediate production of zinc dithionite from the zinc metal dust. Referring to FIG. 1, a commercial metal zinc dust is suspended in an aqueous suspension in line 10 and charged to a zinc dithionite reaction zone 101. Typically, the commercial zinc dust comprises fine particles of metallic zinc of a particle size less than about 75 microns. Although the distribution of zinc particle size will vary and may contain a small amount of particles greater than 75 microns, preferably, less than 2 percent by weight of the zinc dust particles have a particle size greater than 75 microns and less than 250 microns, and more preferably, less than 1 percent by weight of the zinc dust particles have a particle size greater than 75 microns and less than 250 microns. Generally, the commercial zinc dust will contain some impurities including metallic oxides such as zinc oxide and other metallic compounds of metals selected from the group consisting of lead, iron, cadmium, indium, bismuth, copper, zinc, and mixtures thereof. Generally, the zinc oxide content of commercial zinc dust will be less than about 1 weight percent.

[0038] The zinc dithionite reaction zone 101 is a stirred reactor. A zinc metal stream in line 10 and an aqueous stream in line 15 are charged to the zinc dithionite reaction zone 101 to form an aqueous zinc suspension and a sulfur dioxide stream in line 80 is bubbled through the zinc dithionite reaction zone 101 to chemically convert the suspended zinc metal dust to zinc dithionite. According to the invention, a small amount of promoter compound in line 90, such as a zinc compound selected from the group consisting of zinc oxide, zinc sulfite, zinc bisulfite, and mixtures thereof, is charged to the zinc dithionite reaction zone 101 either before or at the same time the sulfur dioxide is introduced to the zinc dithionite reaction zone 101. Preferably, the effective amount of promoter compound charged to the zinc dithionite reaction zone ranges from about 2 to about 15 percent by weight of zinc metal; more preferably, the effective amount of promoter compound charged to the zinc dithionite reaction zone ranges from about 3 to about 9 percent by weight of zinc metal, and most preferably, the effective amount of promoter compound charged to the zinc dithionite reaction zone ranges from about 4 to about 6 percent by weight of zinc metal. The sulfur dioxide chemical reaction of the zinc metal with the sulfur dioxide is exothermic and is operated at a sulfur dioxide reaction temperature of from about 20° C. and maintained below about 54° C. As the sulfur dioxide reaction proceeds, and as the sulfur dioxide is continuously introduced to the zinc dithionite reaction zone 101 at a fixed rate, the pH of the reaction mixture will gradually drop from an initial pH of about 6-8 to about a 3.3 pH. The zinc dithionite reaction is terminated when the pH reaches the terminal pH value of about 3.3 or the sulfur dioxide addition rate is reduced to maintain the pH at about 3.3. According to the invention, the addition of promoter compound in line 90 at a level of between about 2 to about 10 weight percent based on the zinc metal was found to significant improve the cycle time over which the sulfur dioxide reaction can be carried out, while unexpectedly improving the yield of zinc dithionite. Without the zinc compound being present in an amount greater than about 2 weight percent of the zinc metal, the rate at which the sulfur dioxide in line 80 can be introduced to the zinc dithionite reaction zone 101 had to be periodically adjusted to progressively lower levels over long periods of time per cycle in order to achieve the desired conversion to zinc dithionite. The present invention not only permits fewer adjustments to the rate at which the sulfur dioxide is introduced, but also achieves the same or higher conversion to zinc dithionite in a substantially shorter cycle time. At the conclusion of the zinc dithionite reaction, the contents or effluent of the zinc dithionite reaction zone 101 are passed to a sodium dithionite reactor 102 via line 20.

[0039] In an alternate or parallel operation, at least a portion and up to essentially the entire contents of the zinc dithionite reaction zone 101 after any degree of conversion may be passed via line 22 to a zinc filtration zone 106 to recover a zinc dithionite filtrate in line 28 and a solid zinc cake in line 24. The solid zinc cake can optionally be caustic washed in a conventional manner which included further washing the cake to remove residual caustic (not shown) to remove any impurities and returned to the zinc dithionite reaction zone 101 via line 26. A residue of sodium hydroxide in the solid zinc cake was found to have a beneficial effect. It was discovered that the addition of sodium hydroxide as a promoter compound to the zinc dithionite reaction zone 101, prior or simultaneously with the addition of sulfur dioxide, stabilized the zinc dithionite which was formed by the reaction with the sulfur dioxide. It is believed that the sodium hydroxide reacted with the sulfur dioxide to form sodium sulfite, sodium bisulfite, sodium metabisulfite, and mixtures thereof, thereby maintaining the thiosulfate concentration at a minimum level at the start of the reaction, while maintaining the pH of the zinc dithionite reaction zone at a high level (preferably greater than about 4.0). At least a portion of the solid zinc cake may be removed from the process in line 25 to minimize the buildup of impurities in the zinc dithionite reaction zone 101.

[0040] In the sodium dithionite reactor 102, the zinc dithionite reactor effluent passed via line 20 and, optionally the zinc dithionite filtrate in line 28 is contacted with a dilute caustic solution of sodium hydroxide in an amount sufficient to convert the zinc dithionite to sodium dithionite. Generally, 2 moles of caustic are required for each mole of zinc dithionite to produce the sodium dithionite. The conditions of the conversion of the zinc dithionite to sodium dithionite are well-known to those skilled in the art. At the conclusion of the sodium dithionite reaction, a sodium dithionite stream in line 30 is passed to a solids separation zone 103 such as a filtration zone or a centrifuge zone to recover the sodium dithionite solution product stream in line 40 and to remove a crude zinc oxide solid stream in line 50. The crude zinc oxide solid stream in line 50 is passed to a wash zone 104, wherein the crude zinc oxide solid stream in line 50 is washed, preferably with water, to remove any remaining sodium dithionite and to provide a washed zinc oxide stream in line 60. Typically, the crude zinc oxide is washed with a wash water stream. The washed zinc oxide stream in line 60 is passed to a drying zone 105 to dry the washed zinc oxide stream to provide a dry zinc oxide stream in line 70. In some process arrangements, the wash zone 104 and the and the solids separation zone 103 may be carried out in the same equipment such as a filtration zone wherein the recovered filter cake is washed in the same equipment. The dry zinc oxide stream in line 70 or a portion of the washed zinc oxide stream in line 60 may be further refined by any chemical or electrochemical means or sold.

[0041] Referring to FIG. 2, another embodiment of the present invention is illustrated wherein the zinc dithionite reaction zone 201 is charged with an aqueous zinc metal suspension comprising water and a zinc metal in line 110 which is produced by the electrolysis of a portion of a washed zinc oxide stream recovered from a sodium dithionite reaction. According to FIG. 2, the aqueous zinc metal suspension in line 110 is passed to the zinc dithionite reaction zone 201 which is stirred and operates in a batch or semi batch mode. In the zinc dithionite reaction zone 201, the zinc metal is contacted with a sulfur dioxide stream introduced in line 125 in the presence of a zinc compound introduced in line 120. The promoter or zinc compound is introduced at a level of about 2 to about 15 weight percent based on the amount of zinc metal in the zinc dithionite reaction zone 201. The sulfur dioxide stream in line 125 is bubbled through the zinc dithionite reaction zone at a constant rate or a series of successively reduced rates until the contents of the zinc dithionite reaction zone 201 approaches a terminal pH of about 3.2 at the lowest sulfur dioxide addition rate. During the zinc dithionite reaction, the zinc dithionite reaction temperature is maintained below about 54° C., and more preferably, during the zinc dithionite reaction, the zinc dithionite reaction temperature is maintained from about 20 to about 54° C. A zinc dithionite solution is removed from the zinc dithionite reaction zone 201 and passed via line 115 to a sodium dithionite reaction zone 203 via lines 115 and 130 wherein the zinc dithionite is contacted with a dilute caustic solution introduced via line 145. The dilute caustic solution in line 145 typically contains sodium hydroxide, and is introduced to the sodium dithionite reaction zone 203 at effective conditions to convert the zinc dithionite to sodium dithionite. In the sodium dithionite reaction zone 203, zinc oxide is produced as an insoluble solid byproduct. Following the formation of sodium dithionite, the effluent or in the case of a batch operation, the contents of the sodium dithionite reaction zone 203 are passed in line 150 to a solid separation zone 204. In the solid separation zone 204, at least a portion of the sodium dithionite solution is separated from any solids and a sodium dithionite product solution is withdrawn in line 160. The solid separation zone 204 may be a filtration zone or a centrifuge zone to separate the aqueous phase product stream and recover a crude zinc oxide byproduct. The crude zinc oxide byproduct is passed via line 155 to a wash zone 205 where the crude zinc oxide byproduct is washed, typically with water, to provide a washed zinc oxide product in line 170. Spent water is withdrawn from the wash zone 205 in line 175′. In some process flow arrangements, the solid separation zone and the wash zone 205 may be contained in the same apparatus. For example, when the solid separation zone 204 is a filtration zone to separate the crude zinc oxide byproduct from the sodium dithionite solution, the resulting filter cake containing the crude zinc oxide can be washed to provide the washed zinc oxide product in line 170. A portion of the washed zinc oxide byproduct is withdrawn via line 170 for conventional drying and sales. At least a portion of the washed zinc oxide byproduct presscake, or zinc mud stream comprising zinc oxide, in line 172 is passed via line 174 where is combined with a water stream comprising caustic as an electrolyte stream in line 180 to provide a zinc oxide slurry in an electrochemical reaction zone 206. A portion of the washed zinc oxide byproduct presscake can be employed as a portion of all of the promoter or zinc compound in line 120. The electrochemical reaction zone 206 comprises an undivided electrochemical reaction zone having electrodes wherein the aqueous zinc slurry is electrolyzed to convert at least a portion of the zinc oxide into a zinc metal suspension. The electrodes may comprise a copper, magnesium, nickel, or stainless steel or other suitable material as a cathode and a nickel or stainless steel anode. A makeup zinc oxide stream in line 185 may be added to the electrochemical reaction zone 206 to maintain the level of zinc in the process. A zinc metal suspension in line 190 is withdrawn from the electrochemical reaction zone 206 and passed to a zinc metal solids separation zone 207, wherein the solid zinc metal is recovered and passed via line 200 to a second wash zone 208. The zinc metal solids separation zone 207 and the second wash zone 208 can be a single filtration zone wherein the resulting filter cake comprising the solid zinc metal is washed in the same filtration zone. Wash water introduced in line 165 is employed to wash the zinc metal filter cake and spent wash waster is withdrawn in line 175′. A liquid portion of the effluent from the zinc metal solid separation zone 207 is withdrawn in line 195 and reused as a water stream to form the zinc slurry in the electrochemical reaction zone 206. In the zinc metal wash zone 208, the recovered solid zinc metal is washed with water to remove any residual caustic, and the washed zinc metal is produced as an aqueous zinc metal suspension in line 110. The aqueous zinc metal suspension in line 110 is passed to the zinc dithionite reaction zone 201. Thus, this embodiment of the present invention provide an environmentally safe processing scheme for the production of sodium dithionite, with the ability to reprocess the zinc oxide produced in the sodium dithionite reaction step. It was unexpectedly discovered that the presence of the promoter compound in the zinc dithionite reaction zone 201 significantly reduces the cycle time of the zinc dithionite reaction zone while providing an overall improved yield and recovery of zinc dithionite.

[0042] In a further embodiment of the present invention which recognizes the problem of the build-up or accumulation of zinc sulfide in the process, at least a portion of the zinc dithionite solution in line 130 is optionally or periodically passed in line 130′ to a zinc sulfide filtration zone 202 to remove essentially all or at least a portion of the zinc sulfide in line 140, prior to returning a zinc sulfide reduced stream in line 135 to the sodium dithionite reaction zone 203. By the term zinc sulfide reduced stream it is meant that the portion of the zinc dithionite solution in line 135 contains a lower concentration of zinc sulfide than unfiltered or bypassed zinc dithionite solution in line 130. Although removal of the zinc sulfide from the process can have an adverse effect on the operation of the electrochemical reaction zone 206, it was discovered that high current efficiency and the reactivity of the zinc metal particles produced in the electrochemical reaction zone 206 are maintained by only removing a portion of the zinc sulfide from the zinc dithionite solution in line 115. Zinc sulfide and other insoluble materials including zinc particles collected in the solid zinc cake in line 24 can be employed with or without further purification as a promoter in the zinc dithionite reaction zone 201, or in the electrochemical reaction zone 206. The presence of such materials in the zinc dithionite reaction zone 201, will partially limit or control the amount of impurities generated in the zinc dithionite reaction zone and permit the extension of the terminal pH value of the zinc dithionite to a level higher than 3.3. Preferably, the presence of a zinc promoter in the zinc dithionite reaction zone will permit the extension of the terminal pH value of the zinc dithionite to a level higher than about 4.0, and more preferably, the presence of a zinc promoter in the zinc dithionite reaction zone will permit the extension of the terminal pH value of the zinc dithionite reaction to a level higher than about 4.2.

[0043] The following specific examples will provide detailed illustrations of the methods of producing and utilizing compositions of the present invention. These examples are not intended, however, to limit or restrict the scope of the invention in any way and should not be construed as providing conditions, parameters or values which must be utilized exclusively in order to practice the present invention. Unless otherwise specified, all parts and percents are by weight.

EXAMPLE Example 1

[0044] General Procedure:

[0045] Particulate zinc material (206.3 g of commercial zinc dust P-65, available from Noranda Zinc, Toronto, Canada) and water (700 to 1000 g) was charged into a one-liter jacketed resin kettle equipped with a thermometer, nitrogen inlet and outlet adapter, a gas disparager, mass flow meter to measure the amount of sulfur dioxide passed, a pH probe, and a mechanical stirrer. The reaction mixture was then stirred and sulfur dioxide gas was passed through the reaction mixture. The sulfur dioxide was added at a rate of 3.4 g/minute until pH of the reaction mixture dropped to 3.3 and temperature was maintained at 25 to 55° C. by circulating cold water through the jacket. Sulfur dioxide addition was stopped when pH of the reaction mixture dropped to 3.3 and zinc dithionite solution was decanted to give a translucent solution of the zinc dithionite. It was then titrated to determine the concentration of zinc dithionite.

Example 2

[0046] The results of the reaction with sulfur dioxide under various conditions are shown in Table 1. TABLE 1 Zinc Agitation Moles Conv. Reaction Exp. Zinc Dust Rate Of Of zinc Yield of time, No. dust g Water g moles ZnO g rpm ZnS₂O₄ dust³ % ZnS₂O₄ % Minutes 7I-16 206.3 714 3.15  0 560 2.1 — 66.7 83 7I-15 206.3 714 3.15 19.7¹ 560 2.49 82.7% 79.0 97 7I-19 206.3 906 3.15 20¹ 560 2.42 84.2 76.8 97 7I-18 206.3 906 3.15  0 560 2.33 78.1 74.0 92 7I-39⁴ 206.3 906 3.15  0 560 2.30 79.1 73.0 92 7I-54³ 206.3 906 3.15 22¹ 560 2.53¹ 86.3 80.3 133 7I-40⁴ 206.3 906 3.15 22¹ 560 2.52 85.4 80.3 106 7I-49⁴ 206.3 906 3.15 14¹ 560 2.56 84.7 81.3 102 7I-50³ 206.3 906 3.15 10¹ 560 2.50 86.1 79.4 104 7I-53 206.3 906 3.15  4¹ 560 2.43 81.9 77.1 92 7I-51³ 206.3 906 3.15  6¹ 560 2.48 84.8 78.7 93 7I-46³ 206.3 906 3.15 30¹ 560 2.60 85.4 82.5 106 7I-44³ 206.3 906 3.15  0 560 2.60² 87.9 82.5 146 7I-41³ 206.3 906 3.15 22¹ 560 2.72² 94.6 86.4 133 7I-53 206.3 906 3.15  4¹ 560 2.43 81.9 77.1 92 7I-45³ 206.3 906 3.15 14¹ 560 2.71² 92.2 86.0 128 7H-25 206.3 906 3.15  0 560 2.32 79.2 73.6 86 7H-19 206.3 906 3.15 22⁴ 560 2.68 93.6 85.1 115

Example 3

[0047] General Procedure:

[0048] In these experiments, a resin Kettle (5 inch (12.7 cm) in diameter and 18 inch (45.7 cm) high) was used as the cell. A solution or slurry of zinc oxide in the aqueous sodium hydroxide solution (3 to 3.5 liters) at 20 to 80° C. was charged into the resin kettle. Zinc oxide was also added during the experiment to maintain a maximum concentration of solubilized zinc oxide. A thermometer, desired cathodes and anodes were positioned in the cell using laboratory clamps. Mixing was achieved by bubbling nitrogen and by using a mechanical stirrer. Parts of the cathode and anode surfaces were covered with Teflon or electrical tape to achieve the desired active cathode and anode surface areas. Electrolysis was carried out at a current density of about 5000 Amps/m². A portion of the zinc deposited on the cathode was removed periodically. At the end of the experiment, zinc particles were separated from the electrolyte by decantation, and washed with water. Wet zinc (A grams) was charged into a one-liter resin kettle equipped with a thermometer, nitrogen inlet and outlet adapter, a gas disparager, mass flow meter to measure the amount of sulfur dioxide passed, a pH probe, and a mechanical stirrer. An amount (X ml of 0.18%) of an aqueous solution of cadmium sulfate was then charged into the resin kettle. The reaction mixture was then heated to 43° C. and sulfur dioxide gas was passed through the reaction mixture. The sulfur dioxide addition rate was controlled to maintain pH of the reaction mixture above 3.3 and temperature was maintained at 25 to 55° C. Initial addition rate of sulfur dioxide was 3.4 g/minute. Zinc dithionite solution was decanted to give a translucent solution of the zinc dithionite. It was then titrated to determine the concentration of zinc dithionite.

Example 4

[0049] The results of electrolysis of zinc oxide followed by the reaction with sulfur dioxide under various conditions are shown in Table 2. TABLE 2 ZnO¹ Reaction Moles Exp. NaOH Source/ X ZnO^(1,2) Cathode/ Time of e⁻¹ Moles C.E. No. (wt %) wt % A g ml Source/g Anode Min. passed ZnS₂O₄ (%) 7F-36^(3,4) 25 W/6.0 1015 8 0 Cu/Ni 133 6.32 2.20 70 7F-37^(3,4) 25 W/6.0 1121 8 F/10 Cu/Ni 124 6.32 2.84 90 7G-1⁴ 25 F/6.0 926 8 F/10 Cu/Ni 159 6.32 2.57 81 7G-3⁴ 25 F/6.0 946 8 0 Cu/Ni 197 6.32 2.28 72

Example 5

[0050] Particulate zinc material (206.3 g), zinc sulfite dihydrate (25.0 g), water (906 g) were charged into a one-liter jacketed resin kettle equipped with a thermometer, nitrogen inlet and outlet adapter, a gas disparager, mass flow meter to measure the amount of sulfur dioxide passed, a pH probe, and a mechanical stirrer. The reaction mixture was then stirred and sulfur dioxide gas was added at a rate of 3.4 g/minute for 100 minutes. During this period, pH of the reaction mixture dropped to 3.3. Temperature of the reaction mixture was maintained at 25 to 43° C. by circulating cold water through the jacket. The resulting reaction mixture was decanted to give a translucent solution of the zinc dithionite. It was then titrated to determine the concentration of zinc dithionite. Yield of zinc dithionite was 83.2% and conversion of zinc particles was 86.4.

Example 6—Comparative Example Cycle Time

[0051] With reference to Table 1, Experiment 71-44 illustrated the operation of a zinc dithionite reaction without the presence of any promoter compound in the zinc dithionite reaction zone and Experiment 7H-19 illustrated the operation of the zinc dithionite reaction in the presence of about 6.5 weight percent zinc oxide. The presence of the zinc oxide resulted in a decrease in the cycle time, or reaction time for the zinc dithionite reaction by about 31 minutes, by surprisingly reducing the required reaction time from 146 minutes to 115 minutes, or about a 21 percent reduction in cycle time, accompanied by an unexpected 5.7% increase in the conversion of the zinc dust to 93.6 percent, and bringing the overall yield of zinc dithionite to 85.1 percent.

Example 7

[0052] Production of Sodium Dithionite with Removal of Insolubles from Zinc Dithionite Solution

[0053] A resin kettle (5 inch (12.7 cm) in diameter and 18 inch (45.7 cm) high) was used as an electrolytic cell. A solution of zinc oxide (KADOX-920P, Zinc Corporation of America) (299 g), and sodium hydroxide (1248 g) in water (3444 g) was charged to the resin kettle. Zinc oxide (189 g) was also added in installments during the experiment to maintain a maximum concentration of solubilized zinc oxide, according to the instant invention. A thermometer, 2 nickel based anodes and one magnesium based cathode, were positioned in the cell using laboratory clamps. Mixing was achieved by using a mechanical stirrer. Parts of the cathode and anode surfaces were covered with Teflon and electrical tape to achieve the desired active cathode and anode surface areas. Electrolysis was carried out at a current density of about 5000 Amps/m². A portion of the zinc deposited on the cathode was removed periodically. At the end of the experiment, zinc metal particles were separated from the electrolyte by decanting, and were washed with water.

[0054] A zinc dithionite reaction mixture of wet zinc metal (223.1 grams), zinc oxide (KADOX-920P) (10 g), and water (882 g) were charged into a one-liter resin kettle equipped with a thermometer, nitrogen inlet and outlet adapter, a gas disparager, mass flow meter to measure the amount of sulfur dioxide passed, a pH probe, and a mechanical stirrer. Sulfur dioxide gas was then passed through the zinc dithionite reaction mixture at a rate of 3.4 g/minute to produce a zinc dithionite solution. The sulfur dioxide addition rate was lowered several times to maintain pH of the reaction mixture above 3.3 and temperature was maintained between 25 and 55° C. until the reaction was terminated at a pH of about 3.3. The zinc dithionite solution was mixed with filter cell (20 g), filtered, and washed with water to provide a filtered solution of zinc dithionite (2.19 moles; current efficiency of 69.6%) and a wet cake of insolubles (42 g).

[0055] The zinc dithionite solution was further reacted with sodium hydroxide to produce sodium dithionite. A solution of sodium hydroxide (235 g) in 1209 g water was charged into a four neck round bottom flask equipped with a mechanical stirrer, nitrogen inlet and outlet adapters, and an addition funnel. The filtered zinc dithionite solution was then added slowly into the sodium hydroxide solution while controlling temperature below 45° C. Reaction mixture was stirred for 10 minutes, filtered, and washed with water to provide a solution of sodium dithionite and a wet cake of zinc oxide (440.1 g).

Example 8

[0056] A Portion of Zinc Dithionite Insolubles Added to Electrolyte).

[0057] The procedure of Example 7 was repeated with the addition of a portion of the insolubles recovered from the zinc dithionite solution in Example 7 being added to the electrolytic cell. A resin Kettle (5 inch (12.7 cm) in diameter and 18 inch (45.7 cm) high) was used as the cell. Electrolyte—prepared by mixing the decanted electrolyte from the control run, a zinc oxide wet cake (440 g) produced during the hydrolysis of zinc dithionite in the control run, a wet cake of insolubles (38 g) produced in the control run, and 100 g of sodium hydroxide—was charged into the resin kettle. A thermometer, 2 nickel based anodes and one magnesium based cathode, were positioned in the cell using laboratory clamps. Mixing was achieved by using a mechanical stirrer. Parts of the cathode and anode surfaces were covered with Teflon and electrical tape to achieve the desired active cathode and anode surface areas. Electrolysis was carried out at a current density of about 5000 Amps/m2. A portion of the zinc deposited on the cathode was removed periodically. At the termination of the electrolysis step, zinc metal particles were separated from the electrolyte by decanting, and were washed with water.

[0058] The zinc metal was converted to a zinc dithionite solution as follows. Wet zinc (353 grams), zinc oxide (KADOX-920P) (10 g) and water (747 g) were charged into a one-liter resin kettle equipped with a thermometer, nitrogen inlet and outlet adapter, a gas disparager, mass flow meter to measure the amount of sulfur dioxide passed, a pH probe, and a mechanical stirrer. Sulfur dioxide gas was then passed through the reaction mixture at a rate of 3.4 g/minute. The sulfur dioxide addition rate was lowered several times to maintain pH of the reaction mixture above 3.3 and temperature was maintained between 25 to 55° C., until the zinc dithionite reaction was terminated at a pH of 3.3. Recovery of the zinc dithionite solution comprised mixing the zinc dithionite solution with filter cell (20 g), filtering the zinc dithionite solution and washing the filter with water to provide a zinc dithionite solution (2.81 moles; current efficiency of 89.2%) and a wet cake of insolubles (45 g).

[0059] The zinc dithionite solution was converted to a sodium dithionite solution as follows. A solution of sodium hydroxide (257 g) in 1000 g water was charged into a four neck round bottom flask equipped with a mechanical stirrer, nitrogen inlet and outlet adapters, and an addition funnel. The above produced filtered zinc dithionite solution was then added slowly to the sodium hydroxide solution while maintaining the temperature in the flask below 45° C. In order to maintain pH above 12.0, 70 g of a 16% solution of sodium hydroxide in water was added during the hydrolysis. Reaction mixture was then stirred for 10 minutes, filtered, and washed with water to provide a solution of sodium dithionite and a wet cake of zinc oxide (498.6 g). Thus, the addition to the electrolytic cell of at least a portion of the insolubles recovered from the zinc dithionite solution, which are believed to comprise zinc sulfide, resulted in a significant improvement in the current efficiency or yield of both the zinc dithionite and the sodium dithionite. When electrochemically produced zinc is the only source of zinc included in the zinc dithionite reaction mixture, the current efficiency is equal to the yield of zinc dithionite.

Example 9

[0060] Sodium Hydroxide Promoter

[0061] Particulate zinc material (206.3 g), water (906 g) were charged into a one-liter jacketed resin kettle equipped with a thermometer, nitrogen inlet and outlet adapter, a gas disparager, mass flow meter to measure the amount of sulfur dioxide passed, a pH probe, and a mechanical stirrer. Prior to the introduction of the particulate zinc, sodium hydroxide was added to the water to bring the water to the indicated sodium hydroxide level, expressed as a weight percent of the zinc. The reaction mixture was then stirred and sulfur dioxide gas was added at a constant rate of 3.4 g/minute for about 100 minutes. The resulting yield of zinc dithionite at various levels of sodium hydroxide addition between 0 and about 9 weight percent is shown in Table 3. A significant increase in the yield of zinc dithionite resulted for those runs when sodium hydroxide was present in an amount up to about 9 weight percent. TABLE 3 EFFECT OF THE ADDITION OF SODIUM HYDROXIDE Gassing Time, ZnS₂O₅ ZnS2O3 Yield of Exp. No. NaOH % min moles moles ZnS₂O₄ % 7H-BKB-18 0 100 0.085 0.016 74 1-BAC-25 2 95 0.123 0.022 77 1-BAC-28 5 98 0.162 0.029 83 7I-BKB-47 6 107 0.171 0.01 84 1-BAC-31 9 96 0.212 0.024 79

Example 10

[0062] Sodium Hydroxide with Sulfur Dioxide at a Variable Rate

[0063] The procedure of Example 9 was repeated for a variable sulfur dioxide addition rate until the reaction vessel reached a terminal pH value of about 3.3, where upon the sulfur dioxide addition rate was reduced in a step-wise manner to maintain the pH about 3.3 for a total period of about 150 minutes. Runs were made using 0 and 6 grams of sodium hydroxide in the 906 grams of water. The resulting yield of zinc dithionite when no sodium hydroxide was present was 83 percent, compared to a yield of 89 percent zinc dithionite when the sodium hydroxide was present at a level of about 3 weight percent of the zinc.

[0064] It must be noted that as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise. Unless defined otherwise all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs.

[0065] While the invention has been explained in relation to its preferred embodiments, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading the specification. Therefore, it is to be understood that the invention disclosed herein is intended to cover such modifications as fall within the scope of the appended claims. 

I claim:
 1. A process for preparing a zinc dithionite solution which comprises reacting an aqueous slurry of zinc metal with sulfur dioxide in presence of an effective amount of a promoter compound.
 2. The process of claim 1 wherein the reacting step is maintained at a pH above about 3.3 and a reaction temperature below about 54° C.
 3. The process of claim 1 wherein the promoter compound comprises an alkali metal hydroxide and the reacting step is maintained at a pH above about 4.0.
 4. The process of claim 1 wherein the sulfur dioxide is introduced to the aqueous slurry of zinc metal at a constant rate and the promoter compound is admixed with the aqueous slurry of zinc metal either before or after the sulfur dioxide is introduced.
 5. The process of claim 3 wherein the constant rate of introducing sulfur dioxide is adjusted in a series of constant rate increments to maintain pH above about 3.2.
 6. The process of claim 1 wherein the zinc metal comprises particulate zinc material and/or zinc powder.
 7. The process of claim 1 wherein the promoter compound comprises at least one compound selected from the group consisting of an alkali metal hydroxide, zinc oxide, zinc hydroxide, an alkali metal zincate, zinc sulfite, zinc bisulfite, zinc metabisulfite, an alkali metal sulfite, an alkali metal bisulfite, and mixtures thereof.
 8. The process of claim 1 further comprising electrolyzing an alkaline suspension or slurry of a zinc mud comprising zinc hydroxide in an aqueous alkaline suspension to provide the aqueous slurry of zinc metal.
 9. The process of claim 8, wherein the electrolyzing step takes place in an electrochemical cell having cathodes based on metals selected from the group consisting of copper, magnesium, stainless steel, and nickel.
 10. The process of claim 9 wherein the electrochemical cell comprises an undivided electrochemical cell having a copper cathode and a nickel anode.
 11. The process of claim 9 wherein the electrochemical cell comprises an undivided electrochemical cell having a copper cathode and a stainless steel anode.
 12. The process of claim 8, wherein the electrolyzing step is carried out at a temperature of about 20 to about 65° C.
 13. The process of claim 1, wherein the effective amount of the promoter compound comprises 2% to 15% by weight of the zinc metal.
 14. The process of claim 1, wherein the effective amount of the promoter compound comprises 3% to 9% by weight of the zinc metal.
 15. The process of claim 1, wherein the effective amount of the promoter compound comprises 4% to 6% by weight of the zinc metal.
 16. A process for the production zinc dithionite solution, said process comprising: a) introducing a sulfur dioxide stream to a reaction zone containing an aqueous slurry of zinc metal in presence of a promoter compound comprising a compound selected from the group selected from the group consisting of an alkali metal hydroxide, zinc oxide, zinc hydroxide, an alkali metal zincate, zinc sulfite, zinc bisulfite, zinc metabisulfite, an alkali metal sulfite, an alkali metal bisulfite, and mixtures thereof; b) reacting the zinc metal with the sulfur dioxide to produce a zinc dithionite effluent comprising a zinc dithionite solution and insolubles; and c) recovering the zinc dithionite solution.
 17. The process of claim 16 wherein the promoter compound is introduced to the reaction zone prior to or simultaneously with the passing sulfur dioxide stream.
 18. The process of claim 16 further comprising recovering and returning at least a portion of the insolubles to the reaction zone.
 19. The process of claim 16 further comprising passing at least a portion of the zinc dithionite reactor effluent at effective sodium dithionite reaction conditions to a sodium dithionite reaction zone and therein contacting a dilute caustic solution to provide a sodium dithionite product solution.
 20. The process of claim 16 further comprising removing at least a portion of the insolubles from the zinc dithionite effluent and returning at least a portion of the insolubles to an electrochemical cell for producing the aqueous slurry of zinc metal.
 21. A process for preparing a sodium dithionite product solution comprising: a) admixing a zinc mud stream comprising zinc oxide with an electrolyte stream comprising water and caustic in an undivided electrochemical reaction zone and electrolyzing said reaction zone to provide a zinc metal suspension; b) recovering a zinc metal stream from the zinc metal suspension; c) washing the zinc metal stream to provide an aqueous zinc metal suspension; d) admixing a promoter compound with water in a zinc dithionite reaction zone to provide an aqueous zinc compound suspension; e) admixing the aqueous zinc metal suspension with the aqueous zinc compound suspension to provide a mixed zinc suspension; f) contacting the mixed zinc suspension with a sulfur dioxide stream in a zinc dithionite reaction zone and reacting the mixed zinc suspension with the sulfur dioxide to provide a zinc dithionite solution and insolubles; g) contacting the zinc dithionite solution with a dilute caustic solution in a sodium dithionite reaction zone to provide a sodium dithionite effluent stream comprising sodium dithionite solution and a solid zinc hydroxide material; h) separating the solid zinc hydroxide material from the sodium dithionite effluent stream to provide a recovered zinc oxide stream and the sodium dithionite product solution; i) washing the recovered zinc oxide stream to provide a washed zinc oxide stream; j) combining at least a portion of the washed zinc oxide stream with the zinc oxide feed stream to provide the zinc mud stream; k) combining at least a portion of the washed zinc oxide stream with water in step (d), and l) recovering the sodium dithionite product solution.
 22. The process of claim 21 wherein the promoter compound is selected from the group consisting of an alkali metal hydroxide, zinc mud, zinc hydroxide, an alkali metal zincate, zinc oxide, zinc sulfite, zinc bisulfite, zinc metabisulfite, an alkali metal sulfite, an alkali metal bisulfite, and mixtures thereof.
 23. The process of claim 21, further comprising recovering at least a portion of the insolubles and returning at least a portion of said insolubles to the undivided electrochemical reaction zone to improve the electrolyzing step therein.
 24. The process of claim 21, further comprising recovering at least a portion of the insolubles and returning said insolubles to the zinc dithionite reaction zone.
 25. The process of claim 21, further comprising at least partially converting the mixed zinc suspension with the sulfur dioxide in step (f), recovering at least a portion of the zinc dithionite solution and returning unreacted zinc suspension and insolubles to the zinc dithionite reaction zone.
 26. The process of claim 7, wherein the alkali metal comprises sodium.
 27. The process of claim 16, wherein the alkali metal comprises sodium.
 28. The process of claim 22, wherein the alkali metal comprises sodium. 